Traditional lens grinding and polishing technologies can most easily make optical surfaces that are portions of spheres or simple flats. (Strong, John, Procedures in Experimental Physics, Chapter 11, Prentice-Hall, New York (1938).) When a crude spherical surface is rubbed repeatedly and randomly against a matching crude spherical surface, with the interface filled with a slurry of small abrasive particles, irregularities are worn off, and both surfaces become more accurately spherical. The natural ease by which spherical optical surfaces can be made was also expressed by F. Twyman, Prism and Lens Making, Hilger and Watts, London (1952), and by D. F. Horne, Optical Production Technology, Crane Russak, New York (1972).
Many examples can be listed in which the performance of an optical system is improved through the use of one or more non-spherical refractive surfaces. “An aspheric surface can be a powerful design tool for the reduction of residuals or the elimination of primary aberrations (especially distortion, astigmatism, and spherical) which will yield to no other design techniques.” (Smith, Warren J., Modern Optical Engineering, page 351, McGraw-Hill, New York (1966).) But as Smith puts it, “Aspherics, cylinders, and toroids do not share the universality of the spherical surface, and their manufacture is difficult. While a sphere is readily generated by a random grinding and polishing (because any line through the center is an axis), optical aspherics have only [at most] one axis of symmetry. Thus, the simple principle of random scrubbing which generates a sphere must be replaced by other means.
An ordinary spherical optical surface is a true sphere to within a few millionths of an inch. For aspherics this precision can only be obtained by a combination of exacting measurement and skilled hand correction.” (Op. cit., page 413.) “In almost all cases, the designer is restricted to the use of spherical refracting or reflecting surfaces, regarding the plane as a sphere of infinite radius. The standard lens manufacturing processes generate a spherical surface with great accuracy, but attempts to broaden the designer's freedom by permitting the use of nonspherical, or “aspheric”, surfaces lead to extremely difficult manufacturing problems; consequently such surfaces are used only when no other solution can be found.” (Kingslake, R., Lens Design Fundamentals, Academic Press, New York (1978).)
The extra work required to generate and polish an accurate aspheric surface may be worthwhile if that surface can be used as part of a mold, to manufacture large numbers of lenses with the desired non-spherical shape. Aspheric lenses, Fresnel lenses, and diffractive optical elements are commonly manufactured for special system requirements by injection or compression molding of thermoplastic optical polymers such as PMMA, polystyrene, or polycarbonate, or by casting a transparent epoxy or thermoset material in such an aspheric mold. But, unless the optical components are quite thin, such polymers are severely limited in their infrared transmission, typically to less than 1.7 microns wavelength, by molecular resonance bands, and may be limited to 0.300 microns wavelength or longer in the ultraviolet.
Examples of polymer spectral transmission measurements can be found in the USPL Handbook of Plastic Optics (United States Precision Lens, Cincinnati, second edition, p. 20 (1983).) (Also see: http://www.gsoptics.com/custom_optics/charts.html (illustration copied here) and http://www.ircon.com/pdf/wtn100.pdf for transmission data for popular plastics, and FIG. 2)
There are also moldable glasses that can be used for manufacturing aspheric lenses, such as those available from LightPath Technologies, Inc. (Geltech) of Orlando, Fla., but these materials are also severely limited in their infrared and ultraviolet spectral transmission range.
In addition, there are many exotic crystals, alloys, and other materials available that can be ground and polished for use as lenses, many of them transparent across much larger parts of the infrared and ultraviolet spectrum, but these materials are not considered to be suitable for volume manufacture by molding in any of the usual ways, and some of them are quite expensive to obtain as raw material. (Tosi, J. L., Optical Materials: Making the Right Choice in the IR, in The Photonics Handbook, p. 391 ff., Laurin Publishing, Pittsfield, Mass. (2003).) Aspheric optical components can be made from some of these crystalline and amorphous alloy materials by Computer-Numerical-Control diamond surface cutting or grinding, or by skilled use of the older manual grinding and polishing procedures mentioned by Strong (op. cit.) and by Smith (op. cit.), but all of these processes may be too slow and expensive for economical high-volume production.
Tosi (op. cit.) characterized the wide range of physical properties of the useful infrared optical materials, about half of which are chemically alkali or other metal halides. Most of them can be ground and polished optically as if they were glass, but there are significant differences. Some are quite brittle, some fracture easily when their temperature is changed, some corrode the materials in contact with them, some melt at very high temperatures, and some decompose before melting if they are heated. They cannot be molded in the manner of thermoplastic resins and cannot easily be cast in desired shapes.
The only infrared optical materials commonly thought to be “moldable” to non-spherical component shapes are the two comparatively heavy and expensive proprietary chalcogenide glasses, Ge22As20Se68 and Ge20Sb15Se65, available commercially from Umicore, a European company. These materials are chemically similar to AMTIR-1 in Tosi's list.
Consequently, it is a primary object of this invention to provide solid state methods, materials, and apparatus by which lenses, lens blanks, and lens components useful in transmitting ultraviolet, visible, and/or infrared light.
It is another object of this invention to provide methods, materials, and apparatus by which lenses, lens blanks, and lens components can be formed at temperatures within the range from approximately room temperature to less than the melt temperatures of the materials.
It is yet another object of this invention to provide aspheric lenses and lens components fabricated with solid state, low temperature processes from specially prepared powders.
It is another object of this invention to provide protective barriers and methods for protecting lenses formed in the solid state against moisture and other environmental effects.
It is another object of this invention to provide materials that can be ground into powders suitable for forming lenses, lens blanks, and lens components by compressing them into cohesive monolithic solids at temperatures less than their melt temperatures.
Other objects of the invention will, in part, appear hereinafter and, in part, be obvious when the following detailed description is read in connection with the drawings.